It is known that the inter-modulation noise is one of the reasons that make the linearity of the receiver be poor. The inter-modulation noise deteriorates the receiving sensitivity at the receiving terminal.
The inter-modulation noise means that frequency components that are composed by the sum and the difference of harmonic components of the input signals are generated at the output terminal when two or more frequency signals are processed at the same time. The inter-modulation noise is usually generated in non-linear devices such as mixer and amplifier. Since the frequency band of the inter-modulation noise is near the frequency of the main signal, the inter-modulation noise is hardly removed by the band pass filter. As the inter-modulation noise became larger, the linearity of the receiver become deteriorated thereby reducing the receiving sensitivity.
To resolve the above problem, a method of improving linearity of the receiver by increasing a voltage or current applied to low-noise amplifier, intermediate frequency amplifier, etc. of the radio frequency receiver is disclosed in the Korean Patent No.10-0222404.
FIG. 1 is a block diagram showing a receiver disclosed in the Korean Patent No.10-0222404.
As shown in FIG. 1, the conventional receiver is constituted of a controller 111, the first attenuator 112, a low noise amplifier 113, a RF band filter 114, a mixer 116, an oscillator 115, the second attenuator 131, an intermediate frequency amplifier 117, an intermediate frequency band pass filter 118, a demodulator 119, and a signal processor 120.
Now, the operation of the conventional receiver disclosed in the Korean Patent No.10-0222404 will be described with reference to FIG. 1.
The first attenuator 112 attenuates an RF signal received via antenna according to an attenuation control signal ATC2. The low noise amplifier 113 amplifies the RF signal outputted from the attenuator 112, and the RF band filter 114 filters the RF signal of the receiving band and outputs it. The mixer 116 mixes the RF signal and the signal outputted from the oscillator to generate a intermediate frequency signal. The intermediate frequency amplifier 117 amplifies the signal outputted from the mixer 116. The intermediate frequency band pass filter 118 performs down conversion of the frequency of the received RF signal, and the demodulator 119 demodulates inputted signal to output I and Q signals. The signal processor 120 receives I and Q signals outputted from the demodulator 119, generates baseband digital data and voice signal, and detects and outputs RSSI (Received Signal Strength Indicator) and E/Io (signal-to-interference ratio). The controller 111 inputs RSSI and E/Io outputted from the signal controller 120 and generates an attenuation control signal ATT, and the first and the second control signals CLT1 and CLT2 to remove inter-modulation noise by analyzing the signal.
That is, if the linearity of the receiver decreases, the receiver disclosed in the Korean Patent No.10-0222404 improves the linearity of the receiver and the receiving sensitivity by increasing the voltage or current applied to the low noise amplifier 113, the intermediate frequency amplifier 117, and the first and the second attenuator 113 and 131 by generating a control signal from the controller 111. However, the above-described receiver has to be equipped with a separate control circuit to improve the linearity of the receiver thereby causing the complexity of the receiver.